1. Field of the Invention
This invention is related to improved silver catalysts for the production of ethylene oxide, their preparation, and their use in ethylene oxide processes.
2. The Prior Art
Materials consisting of silver upon a support are known to be useful catalysts for the production of ethylene oxide by the controlled incomplete oxidation of ethylene with molecular oxygen. A great variety of modifications have been proposed to improve the activity and selectivity of these catalysts. These modifications have involved, for example, the supports employed, the method of production, the physical form of the silver on the support and the addition of additives to the catalyst.
The alkali and alkaline earth metals and their salts have been repeatedly proposed as additives for various silver ethylene oxide catalysts. Carter, in U.S. Pat. No. 2,125,333, issued Aug. 2, 1938, was among the first to disclose alkali metal addition. He specified the use of "small amounts" of alkali metals, including both sodium or potassium in his silver catalyst. Later patents elaborated on this disclosure. McNamee et al in U.S. Pat No. 2,238,474, issued Apr. 15, 1941, disclosed that while addition of 100 ppm by weight to 24% by weight of sodium improved silver catalysts, these amounts of potassium had a detrimental effect on catalyst performance. Sears, Jr. et al in U.S. Pat. No. 2,615,900, issued Oct. 28, 1952, cited a large number of promoters useful in broad weight ranges. Saken, in U.S. Pat. No. 2,671,764, issued Mar. 9, 1954 disclosed the use of large amounts of alkali metal sulfates. Hosoda et al in U.S. Pat. No. 3,144,416, issued Aug. 11, 1964, also cited a number of promoter materials. Kriger et al in U.S. Pat. No. 3,563,913 issued Feb. 16, 1971, generally disclosed the use of alkali and alkaline earth metals as promoters.
Cusumano in U.S. Pat. No. 3,844,981 disclosed the use of pomoters selected form Groups VII-B, I-B or the iron group of Group VIII of the perodic table of the elements. Belgian Pat. No. 822,857 issued June 6, 1975, also disclosed a large number of pomoters in addition to alkali and alkaline earth promoters. Belgium Pat. No. 819,654, issued Dec. 31, 1974, disclosed the use of thallium in conjunction with another metal or metals. Gardner, U.S. Pat. No. 2,366,724, issued Jan. 9, 1945 teaches the use of unsupported catalysts of silver and thallium selenide. No prior art references are known that teach that thallium in selected amounts can be used as a promotor of silver-based ethylene oxide catalysts.